Prestressed-concrete structures built by the post-tensioning system include an inner-cable structure in which tendons, e.g. prestressing steel wires or steel strands, covered with sheaths are provided within a concrete member. In such an inner-cable structure, when a grout is filled in the sheaths after prestressing has been done, it is extremely difficult to detect the filling condition of the grout.
In compliance with the demand for a reduction in weight of prestressed-concrete box girders, an “outer-cable structure” has been adopted to reduce the wall thickness of box girder members, in which prestressing tendons are provided out of the concrete members in cross-section of the girder (i.e. in the space inside the box girder cross-section).
The use of the outer-cable structure allows the box girder itself to be reduced in weight and also permits a reduction in material cost and a reduction in work volume required for construction, plus enabling the maintenance and repair of prestressing cables, advantageously. In addition, the reduction in weight of the superstructure leads to a reduction in cost of the substructure (the foundation and piers of a bridge) supporting the superstructure.
In the outer-cable structure, a prestressing tendon that is made up of a large number of prestressing steel wires or steel strands is inserted into a sheath to form an outer cable.
As in the case of the inner-cable structure, tensile stress is applied to the tendon at anchorages provided at both ends of the box girder through anchoring devices to induce compressive stress (prestress) in the concrete girder, thereby improving the load-carrying capacity of the whole box girder.
In the outer cable, a steel pipe or a black polyethylene pipe is generally used as a sheath from the viewpoint of durability. The black polyethylene pipe is formed by mixing an ordinary polyethylene component with carbon black or the like to blacken the polyethylene pipe for the purpose of preventing the material from being deteriorated by ultraviolet exposure.
Incidentally, tendons, which are made of steel, must be subjected to anti-corrosion treatment because of its property. To carry out anti-corrosion treatment for the tendon in the sheath of each outer cable installed in a box girder at a bridge construction site, cement milk or a resin- or oil-based filler is injected into vacant spaces in the sheath as a grout.
Above all, cement milk is a strongly alkaline inorganic grout. Therefore, if cement milk is satisfactorily filled to enclose prestressing steel or the like, which is sensitive to stress corrosion, best durability is exhibited, and high reliability is obtained.
In general, a grout is injected into the sheath by a grout pump from a grout injection hole provided in the anchorage at one end of the sheath, and it is judged that filling of the grout has been completed when the grout has reached the anchorage at the other end of the sheath.
On this occasion, the filling condition of the grout injected into the sheath can be confirmed only indirectly by making a visual check as to whether or not excess grout has been discharged from the upper ends of discharge hoses provided at several positions in an intermediate portion of the sheath. There has heretofore been no technique for directly and reliably confirming or inspecting the filling condition of the grout in the sheath.
There are some advantages in adopting the outer-cable structure for a box girder bridge: reduction in weight of the concrete member; reduction in time and labor required for steel assembling and concrete placing operations; ease of replacing prestressing steel; and ease of improving an existing bridge in maintenance plus load-carrying capacity. To allow these advantages to be surely exhibited, a design of high accuracy and a reliable operation are required at each step of the outer cable installing operation. Above all, an operation of injecting a grout into a sheath enclosing a tendon is one of important factors influencing the performance of a structure with outer cables.
The object in injecting a grout into the sheath of an outer cable is to fill vacant spaces in the sheath with a homogeneous grout and to enclose a tendon made of prestressing steel or the like satisfactorily, thereby taking anti-corrosion measures. That is, in the grout injection operation, reliable and elaborate filling is important. In the conventional method, however, a black polyethylene pipe or a steel pipe is used as a sheath pipe, and it is therefore difficult to inspect or confirm the filling condition of the grout either during or after the grouting operation. In particular, the filling condition of the grout cannot readily be confirmed or inspected by visual observation. Accordingly, the conventional method suffers from serious problems in terms of reliability and so forth.
There has also been proposed a method of inspecting the filling condition of the grout in which ultrasonic waves are transmitted from one end of a sheath at an anchorage and received at the other end to detect an abnormality when the filling is insufficient [for example, see Japanese Patent Application Unexamined Publication (KOKAI) No. 4-182568]. There has also been proposed a method in which elastic waves are propagated from the sheath surface, and the filling condition of the grout is detected from the way in which the elastic waves are received [for example, see Japanese Patent Application Unexamined Publication (KOKAI) No. 10-54140]. However, it cannot be denied that any of the conventional methods involves problems in terms of practicality, e.g. difficulty in installing terminals for transmission and reception, and need of an advanced measuring device and a high level of signal analyzing capacity.
Moreover, outer cables are designed so that they are not only disposed in parallel longitudinally in a box girder but also caused to change in direction vertically by deflectors provided in the box girder. This is done to prestress the box girder not only in the longitudinal direction but also in the vertical direction so as to cope with various stresses induced in the whole concrete structure. Recently, however, size of a tendon has been increased so as to reduce costs of labor relative to post-tensioning operation including pre and post works such as placing ducts and injecting grout. The use of outer cables with an increased outer diameter requires a special consideration to be given to the deflector structure. That is, it is desirable to provide a structure in which the cable surface and the contact surface of each deflector should rub against each other smoothly without producing unnecessary frictional force transmitted through contacting pressure occurring at the deflector to the concrete structure. Accordingly, it has been desired eagerly that outer cables should be capable of meeting such a structural demand appropriately.
It will be apparent that the above-described outer cable structure is applicable not only to concrete box girders but also to steel box girders.